This invention relates to a novel method of treating subsurface deposits containing heavy or viscous oil so that it may be recovered using hot fluid displacement techniques.
There exist throughout the world major deposits of heavy oils which, until recently, had been substantially ignored as sources of petroleum since the oils contained therein were not recoverable using ordinary production techniques. For instance, only lately has much interest been shown in the heavy oil deposits of Alberta province in Canada even though the deposits are both close to the surface and represent an estimated petroleum resource upwards of many billion barrels. The expense involved in the production of these oils stems from the fact that they are quite viscous at reservoir temperatures. A viscosity of 10,000 centipoise to several million centipoise characterizes Athabasca crude oil. Unless the deposit is on the surface and the heavy-oil-containing material can be mined and placed in a retort for separation from its matrix, some method of treating the deposit in-situ need be utilized for the realization of any substantial petroleum recovery.
Interwell displacement has been recognized as the most efficient method of in-situ recovery of heavy oils. However, before displacement can commence, a warm and liquid communicating path must be established between wells since viscous oil will not flow at any commercial rate until its viscosity is reduced by heat. In-situ or reservoir heating to try to create this communicating path is generally done by steam stimulation, i.e., injection of steam at above fracturing pressure and subsequent production, on an individual well basis. This process does not result in a well defined heated volume. Since the steam is injected into the formation above fracture pressure, the steam takes the unpredictable path of least resistance in the often unconsolidated strata containing the viscous oils. Consequently, oil which would be recoverable by the present invention is not produced. For these reasons it is a formidable task to recover a substantial percentage of the heavy oil in a selected formation while efficiently utilizing available steam. This invention is intended to provide an effective manner for treating and recovering viscous oils.
A number of methods have been suggested for in-situ thermal recovery of viscous oil deposits.
One of the earliest methods entails the steps of first, drilling a single vertical borehole into the petroleum-bearing formation and then injecting a heated fluid such as steam or water into the formation thereby causing the hydrocarbon to become less viscous and flow. The thusly-heated hydrocarbon is finally pumped from the same vertical borehole. Obviously this method is slow, since there is no mean hydraulic force to continually urge the oil towards the wellbore and no source of heat to maintain it in a liquid, or at least pumpable, state. For these reasons, the proportion of petroleum that can be recovered from a particular formation is quite low.
Another early suggestion, in U.S. Pat. No. 3,349,845, to Holbert et al, provides a somewhat complicated method for recovering viscous oils from shale formations. The process entails first drilling a vertical injection well and thereafter forming a system of vertical fractures which, if desired, may be propped open with sand or other granular solids. A horizontal, or output well, is then drilled to intersect the vertical fracture system. A heated petroleum corridor is established by heating the injection well under a low gas pressure. The heating is continued until a zone at least 40 or 50 feet along the wall of the vertical injection well is created. Holbert et al suggests that the entire stratum between injection and output well can be heated but that is usually neither necessary nor desirable. The fractures are then plugged at the injection well. Plugging provides assurance that the subsequently added displacement fluid, which may be steam, displaces the oil into the output well rather than merely flowing through the fractures.
Holbert et al, although alleging the utility of its disclosed process with respect to tar sands, is apparently quite specific to oil shales and of only minor relevance to tar sands. For instance, vertical fracturing is a required step in the process, and yet U.S. Pat. No. 4,020,901, to Pisio et al, indicates that attempts to fracture tar sand formations in a controllable manner do not meet with success. Vertical fractures often terminate uselessly at the surface. The fractures often tend to "heal" as mobilized viscous petroleum flows through the cracks and cools to its immobile state. Pisio et al, additionally mentions that tar sands frequently underlie intermediate overburden layers which are easily fractured.
The Holbert et al process is not particularly useful at a viscous oil deposit such as that found at Athabasca. Much of the Athabasca tar sands are at a depth too deep to mine and much too shallow to create suitable fractures.
Holbert et al additionally suggests propping open the fractures with some known proppant such as sand. When the stratum under consideration is oil shale, propping is a step which facilitates oil flow. However, in the case of a tar sand which is composed of a viscous oil and sand, the use of sand as a proppant is somewhat akin to "carrying coals to Newcastle." The proppant supply becomes part of the sand matrix and the fracture closes.
Finally, it is generally accepted that fracturing an unconsolidated formation such as by tar sand gives unpredictable results, at least with regard to the orientation of the fracture. On the other hand, consolidated formations, such as the oil shales of Holbert et al, can be fractured with reasonably predictable results. The disclosure in Holbert et al requires knowledge of the fracture's orientation so that the horizontal output well can be drilled to intersect the fractures. Knowledge of fracture orientation is unconsolidated tar sands is not, as a rule, available.
A subsequent development is found in U.S. Pat. No. 3,386,508, to Bielstein et al. This process for recovering viscous crude oils involves sinking a large central well, having a bore diameter of 1 to 10 feet, into a subsurface formation containing oil. A number of injection wells are then slant-drilled to intersect the central well within the subsurface oil-bearing stratum. Steam is then introduced into the injection wells only at the upper end of the stratum. Displaced heated oil permeates the walls at the lower end of the injection wells and passes into the central well where it accumulates and is pumped to the surface.
Bielstein et al does not heat an open horizontal borehole and then plug it as is done in the process of the present invention.
An additional set of related developments is found in U.S. Pat. Nos. 3,994,340; 4,020,901; and 4,037,658, to Anderson et al, Pisio et al, and Anderson respectively. Each produces a heated horizontal corridor by the physical placement of long heat exchangers in the tar sand stratum. The three differ from each other principally in the design of their heat exchangers. Each of these specifications additionally discusses the production problems which are unique to tar sands including the difficulty, mentioned above, of creating and maintaining an effective fracture network. None of the three suggests the straightforward and simple method of treating the petroleum-bearing stratum disclosed herein.
Other methods of attaining corridors of heated viscous petroleum, from which the heated oil can be displaced, are known. For instance, U.S. Pat. Nos. 4,010,799 and 4,084,637, to Kern et al and Todd respectively, teach a process in which a number of vertical wells are drilled down into the oil-bearing stratum, electrodes are inserted into the wells, and a voltage imposed across the electrodes in adjacent wells. Although it is understood that a prototype well involving such a process has been drilled, it is apparent that complete control of a resulting heated chamber position is not readily possible. The electric current will take the path of least resistance irrespective of where the driller would place the chamber. This problem is especially pronounced in areas where oil-bearing formations lie in close vertical proximity to electrically-conductive aquifers.